The present invention relates to motion sensors which detect motion in a room and activate a load, such as lights, coupled to line power.
Motion sensors or occupancy sensors control lights and other electric loads by detecting doppler shifts in transmitted ultrasonic sound. An occupancy sensor would typically transmit ultrasonic sound waves via one or more transmitters which then reflect off of objects in the room and are detected by one or more receivers. A doppler shift of the reflected signals indicates the presence of movement within the room, and can be detected by noting frequency changes from the transmitted frequency. In the absence of a detected doppler shift, an electric load can be switched off, thereby conserving energy when no people are in the room. An example of such a system is shown in U.S. Pat. No. 4,661,720, hereby incorporated herein by reference.
It is desirable to have such an occupancy sensor be able to operate on low voltages to reduce power consumption and yet be able to switch the high voltages of the AC line coupled to a load such as room lights, etc.